<-This is a front-end for google's calculator.
Even if you didn't use this you could read the text typed in zenity, write it to a file or just put in into a variable, and then process it with a calculator program (like "bc"). So for example, you get zenity to read some text, put it into a variable, echo that variable into a file, then bc < that_file. Then put that result into another window. (Also you'd have to delete the temporary file)
So, everytime you want to calculate something you get a small window to put what to calculate, then another one telling you the result.